1. Field of the Invention
This invention relates to systems for processing signals and, more particularly, to systems for converting low-frequency baseband signals to digital formats.
2. Prior Art
In certain applications signals are produced which may vary over a range of a million to one, requiring signal resolutions of one part per million. For example, certain inertial navigation systems require such accuracy. For achieving a dynamic range of a million to one, a conventional parallel analog-to-digital circuit requiring 20 bits per sample would be extremely difficult to fabricate and keep in calibration with aging of components and over a range of temperatures. For these applications, a voltage-to-frequency converter is normally used which generates an output signal having a frequency proportional to the voltage level of an input baseband signal. By counting the frequency of the signal produced by the voltage-to-frequency converter over a finite time interval, a quantity representing the integral of the input baseband signal can be computed. The input baseband signals to be measured in such systems have a frequency range which extends down to dc. Shifts in dc level in the circuits of a system are caused, for example, by temperature changes or by subjection to external radiation. Consequently, any shifts in the dc level within any circuit handling a baseband signal limits the voltage accuracy of a system.